As described in German patent publication 296 14 765, a flow-control valve has a housing extending along an axis and having first and second input ports and respective first and second output ports for respective first and second separate fluid streams and formed between the first input port and second input port with a first valve seat and between the second input port and second output port with a second valve seat offset axially from the first valve seat. Respective first and second valve bodies are axially shiftable between positions blocking and unblocking the respective valve seats. A coupling element connected axially between the valve bodies for joint movement of the valve bodies. A diaphragm connected between the first piston and the housing defines between the diaphragm and the first valve seat an axially inner compartment into which the first input port opens and between the diaphragm and the housing an axially outer compartment.
In this system the housing is formed with a passage having one end opening into the housing between the first valve seat and the first outlet and at an opposite end into the outer compartment. Thus the pressure of the first stream upstream of the first valve seat is applied to the inner face of the diaphragm and the pressure of the first stream downstream of the first valve seat is applied to the outer face of the diaphragm. Thus this differential pressure across the first valve seat controls both of the separate valve assemblies. This is a relatively complex construction.
Another problem with the known systems is that the valve seats and valve bodies fit together in a very precise manner, and they tend to lime up. Significant lime deposits largely incapacitate the valves and require servicing or even replacement of them.